THE president's ADDRESS. 45 



evolution. He suggests that I should" perhaps seek to reconcile 

 the statements by insisting on the distinction between ' diver- 

 gent ' and ' progressive ' evolution." Certainly no such idea 

 of reconciliation was in my mind. I thought I had made my 

 position clear in the succeeding paragraph, in which I remarked 

 that " just as we have learnt to regard individuals as the tempo- 

 rary offspring of a continuous stream of germ-plasm, so we must 

 regard species as the somewhat more permanent but nevertheless 

 temporary offshoots of a continuous line of progressive evolution." 

 As I have evidently not succeeded in my intention, I may 

 perhaps be allowed to make a fresh endeavour to relieve myself 

 of the charge of inconsistency, and to illustrate my meaning once 

 more from the great storehouse of facts derived from the study 

 of sponges. A very large number of species in this group are, 

 as I have already indicated, distinguished from one another by 

 minute but constant differences in the form of the spicules, 

 especially of the microscleres, which have no conceivable value 

 from the point of view of adaptation and have in all probability 

 arisen as mutations. Such specific characters may be afforded 

 by the number and form of the teeth on a chela, or by the 

 ornamentation of some other form of microsclere, which can only 

 be made out under high powers of the microscope. Characters, 

 on the other hand, which are obviously adaptive in nature, such 

 as the arrangement of the skeleton as a whole and its relation 

 to the canal-system, which seem to have developed, if not to 

 have originated, by slow, successive variation under the influence 

 of natural selection, are usually common to many species in the 

 same genus ; they are not, as a rule, specific characters. Hence, 

 though species seem frequently to have arisen as mutations in 

 trivial and non-adaptive characters, evolution on the whole 

 seems to have taken place by a process of progressive evolution, 

 in which mutation has played a comparatively small, though 

 by no means negligible part. In the vast majority of cases a 

 specific character appears to bear much the same relation to 

 the organism that exhibits it as the colour of a motor-car bears 

 to the car itself. The colour has nothing to do with the efficiency 

 of the car, and plays no part in the evolution of motor-cars, but 

 ^t affords a very convenient means of distinguishing one car from 



